Abstract

Earthquake-related fault slip in the upper hundreds of meters of Earth's surface has remained largely unstudied because of challenges measuring deformation in the near field of a fault rupture. We analyze centimeter-scale accuracy mobile laser scanning (MLS) data of deformed vine rows within ±300 m of the principal surface expression of the M (magnitude) 6.0 2014 South Napa earthquake. Rather than assuming surface displacement equivalence to fault slip, we invert the near-field data with a model that allows for, but does not require, the fault to be buried below the surface. The inversion maps the position on a preexisting fault plane of a slip front that terminates ~3 to 25 m below the surface coseismically and within a few hours postseismically. The lack of surface-breaching fault slip is verified by two trenches. We estimate near-surface slip ranging from ~0.5 to 1.25 m. Surface displacement can underestimate fault slip by as much as 30%. This implies that similar biases could be present in short-term geologic slip rates used in seismic hazard analyses. Along strike and downdip, we find deficits in slip: The along-strike deficit is erased after ~1 month by afterslip. We find no evidence of off-fault deformation and conclude that the downdip shallow slip deficit for this event is likely an artifact. As near-field geodetic data rapidly proliferate and will become commonplace, we suggest that analyses of near-surface fault rupture should also use more sophisticated mechanical models and subsurface geomechanical tests.

(A) Colored columns, stacked PDFs of S from each vineyard block for the second-measurement epoch (, middle panels). The width of the column represents the along-strike length-scale of the vineyard block. Black circles, hand measurements of fault-parallel surface displacement made immediately following the earthquake. Gray circles, alinement array measurements 60 days after the event (). (B) Colored columns, stacked PDFs of zu from each vineyard block for the second-measurement epoch (, bottom panels). (C) Coseismic, kinematic slip model from Wei et al. (). (D) Normalized cumulative slip from three coseismic slip models. W, the study by Wei et al. (); J, the study by Ji et al. (); M, the study by Melgar et al. (). Colored rectangles in the shallowest portions are cumulative slip S from all the vineyard blocks in this study, normalized according to the maximum value of each of the kinematic models.